Silk is a secreted, fibrous material that is deposited or spun by an organism. From a biochemical point of view, silk consists of protein threads composed of repeating arrays of polypeptides that contain both discrete crystalline and noncrystalline domains that are oriented around a fiber axis.
Several arthropods, such as spiders, caterpillars mites, mantids, moths, and beetles, produce silk, or silk-like fibers. Insects, as a group, as well as spiders, produce many different types of silks and fibrous proteins, such as fibroins and spidroins. An individual spider may produce as many as nine different types of silks and fibrous proteins, each of which may be composed of more than one type of protein (Kovoor 1987; Haupt & Kovoor 1993). Different silks differ in number as well as in sequence of composing proteins. Although all fibroin and spidroin proteins do comprise several repeats, the repeat structures are species dependent and the amino acid composition, as well as the mechanical characteristics, may vary strongly from silk to silk (Zurovec and Sehnal 2002; Fedic et al. 2003).
Although the domesticated silkworm Bombyx mori is the mainstay of the silk industry, there is a considerable trade in some countries in silk produced by silkworms living “wild.” The most important of these wild silks are those that are known as Tussah. Tussah is the product of several species of silkworm of the genus Antheraea, particularly Antheraea mylitta, indigenous to India, and Antheraea pernyi, which is native to China (Huber 1947; Cook 1984). Although Tussah silk is the most important wild silk in commercial use, there are still other varieties of caterpillars that produce silk. These silks are called wild, because these worms are not capable of being domesticated and artificially cultivated. Some examples are: Antheraea yamamai, Attacus ricini, and Attacus Atlas. 
In recent years, spider silk was receiving more and more interest, mainly due to the excellent mechanical characteristics of this silk. For spiders, one species can make different silk fibers for different purposes, such as dragline silk or major ampullate silk, capture-spiral silk, tubuliform silk, aciniform silk and minor-ampullate silk.
The most investigated type of spider silk is the dragline or major ampullate (MA) silk that is secreted by the major ampullate glands of the spider. The dragline is used to support the spider when constructing a web and to prevent it from falling. This function results in mechanical properties combining a high Young's modulus with a high strength. Due to its size and accessibility, the major ampullate gland has been the focus of most studies.
A second important type of spider silk is the flagelliform, spiral or capture silk. This type of silk is composed of an acidific glycoprotein, secreted from the flagelliform gland, and coated with glue from the aggregate gland, which makes it sticky. The glue is not regarded as silk because it is composed of glycoproteins and other amino acids. The flagelliform silk is exclusively used for the construction of the spiral components of the web. This function results in a fiber that is highly extensible and capable of absorbing the energy of the flying prey without failure. The functional role of the glue is believed to allow for more effective capture of prey.
Minor ampullate (MI) silk is the spider silk that is secreted by the minor ampullate glands and is a strong, non-elastic, deformably stretchable silk used in web formation (Colgin & Lewis 1998).
Another spider silk that is discussed in this text is the egg sac silk that is used to wrap eggs. Vollrath (1992, 2000) mentioned in his representation of the spinning glands associated to its function that the soft inner silk of the egg sac is produced by the aciniform glands (aciniform silk), whereas the tough outer silk of the egg sac is secreted by the cylindrical or tubuliform spinning glands (tubuliform silk). Viney et al. (2000) believes the opposite. The tubuliform glands are only found in female spiders, which makes it more probable that the inner silk is indeed secreted by the tubuliform glands.
Because of its attractive properties (high strength, flexible with good water-absorbing power, soft, good elastic recovery behavior, glossiness, etc.), silk has a wide variety of uses in the apparel, drapery, upholstery and military fields. Natural silk has a long history of use as a textile fiber, and has been used in recent years for medical sutures, blood vessels, artificial skin, tendons and for binding enzymes (Bunning et al. 1994; Kuzuhara et al. 1987). Interest in Antheraea pernyi silk for biomedical applications has recently grown because A. pernyi SF contains the tripeptide sequence of arg-gly-asp (RGD), known as cell adhesive site for mammalian cell culture (Minoura et al. 1995; Pierschbacher & Ruoslahti 1984a, 1984b; Li et al. 2003). Therefore, it has been investigated as a potential biomaterial such as a matrix for the enzyme immobilization and mammalian fibroblast cell culture (Kweon et al. 2001a, 2001b). Silk of the spider Nephila clavipes has been used to help mammalian neural regeneration (Allmeling et al. 2006).
As each silk has its own composition and characteristics, there is a lot of interest in the identification of new silk proteins, opening the possibility for new applications. Surprisingly, we found that spider mites, and particularly Tetranychus urticae, are making silk proteins of which the amino acid composition differs rather strongly from that of classical fibroins and spidroins, especially in the alanine, glycine and serine content. Those differences are found in the global protein composition, as well as in the composition of the repeats.